An electrophotographic image forming apparatus (a printer, for example) includes a fixing device for fixing a toner image on a sheet, which toner image has been formed on the sheet. The fixing device fuses the toner image so that the toner image is fixed on the sheet. One example of such a fixing device may be a fixing device employing a pair of rollers, i.e. a fixing roller and a pressure roller (see Patent Literature 1).
The fixing roller is such a roller member that an elastic layer is formed on a surface of a hollow cored bar made of a metal (such as aluminum). In the cored bar, a halogen lamp is provided as a heat source. On the basis of a signal supplied from a temperature sensor provided on a surface of the fixing roller, a temperature control device controls the halogen lamp to be turned on/off so that a temperature of the surface of the fixing roller is controlled.
The pressure roller is such a roller member that a heat resistance elastic layer (such as silicon rubber) is provided on a cored bar, as a coating layer. The pressure roller is pressed against the surface of the fixing roller, so that a nip region is formed between the fixing roller and the pressure roller due to elastic deformation of the elastic layer of the pressure roller.
The fixing device having such an arrangement receives the sheet on which a toner image has been formed but not fixed yet. The fixing device pressures the sheet in the nip region between the fixing roller and the pressure roller, while carrying the sheet by rotation of these rollers. The toner image on the sheet is fused by heat generated from the surface of the fixing roller. Thereby, the toner image is fixed on the sheet.
The conventional fixing device employing such a pair of rollers, however, has the following problems. When the fixing device is turned on, the temperatures of the fixing roller and the pressure roller are equal to a room temperature. Therefore, it is necessary for the fixing device to secure a warm-up period of time for warming these rollers up to a predetermined temperature. Further, even in a standby state in which the image forming apparatus is carrying out no copy operation, it is necessary to keep the surface of each of the rollers being at the predetermined temperature. In other words, it is necessary to keep on heating the surface of each of the rollers even in the standby state, i.e. even during a period of time in which no copy operation is being carried out. This wastes energy.
In view of the problems, there has been proposed a fixing device for fixing a toner by use of power of a laser beam (see Patent Literature 2), as a technique for efficiently fixing only a toner on the sheet without unnecessarily wasting energy.
According to Patent Literature 2, a fixing device employs a light source including a plurality of low-power semiconductor lasers. The fixing device irradiates a toner image located in a certain region on the sheet with a plurality of laser beams by use of the light source so that the plurality of laser beams overlap each other on the toner image. This compensates a shortage of power to fuse the toner so that the toner is fixed on the sheet.
Patent Literature 2 describes that the technique allows the use of a low-power and low-price semiconductor laser and therefore allows simplification of an arrangement of the fixing device.
Further, Patent Literature 3 discloses an image forming apparatus which can fix a toner image on a sheet by irradiating the toner image with such laser beams whose wavelengths correspond to light-absorption wavelengths of full color toners (Y, M, C) and a black toner (B), respectively. According to the technique, (i) a laser used for each of the toners has an optimum wavelength in terms of the light-absorption wavelength of that toner, and the laser is controlled to irradiate only that toner with its laser beam. With this technique, in a case where four lasers whose wavelengths correspond to the respective toners are merely provided, there arise problems of an increase in cost and an increase in size of the fixing device (see Patent Literature 3).